The UK Govt API http://data.gov.uk/ is a good place to start and study.

Here are the 7 design principles that guided them in the design process

1. Start with needs* 2 Do less 3 Design with data 4 Do the hard work to make it simple 5 Iterate. Then iterate again. 6 Build for inclusion 7 Understand context 8 Build digital services, not websites 9 Be consistent, not uniform 10 Make things open: it makes things better

Here is more about that https://www.gov.uk/design-principles#first